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  1. #1

    GP1200R cold cylinder

    I have exhausted my thoughts on what could be wrong with this ski. This is a 2001 GP1200R that i purchased from someone who told me the carb tuning was off after installing a D plate. The ski is ridiculously clean and doesn't appear to have many hours on it so i don't believe it to have been abused. When i purchased the ski i was able to get it running on the hose and it was at the end of the season so i figured i would go through the normal stuff before putting it on the water.

    What has been done to the ski:
    D plate
    flame arrestors
    fuel pressure balance modification to carbs (drill returns and place jet in the last carb)
    accelerator pump disabled
    110 pilot jets, 120 main jets, 1.5 needle and seat and 95 gram springs
    45 psi pop off pressure
    1 turn out on low speed and 1.5 turns out on high speed needles
    actual mikuni gasket sets
    new NGK plugs
    oil injection is retained

    The problem:
    cylinder 1 nearest the front of the ski runs cold and when i try to run it on the water the ski goes 20-25 mph and just bogs.

    What i have tried:
    Compression is the same between all cylinders, 160 PSI
    I adjusting needles on the carb to pick the cylinder up, no change.
    I have spark on that cylinder but tried swapping the coil between cylinders in case it wasn't firing under compression
    I tried adjusting pop off pressure thinking maybe the signal was different between the carbs for some reason, made the problematic carb pop off pressure 60 PSI and the others 45 PSI, no change

    The engine will sit and idle within reason but even at idle i can feel the cylinder running noticeably colder than the others. A mechanic at the marina has suggested that a failing crank seal between cylinders may cause a double pulse signal to the carb making for something like this to happen but i have not found anyone to have a similar issue.

    I didn't buy any Chinese junk parts but could a needle and seat have an issue to flood a cylinder to the point it hardly wants to fire?

    Anyone with new thoughts to this issue would be welcome.

    Thank you,
    Nicholas

  2. #2
    jeffg426690's Avatar
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    Some thoughts:
    Sounds like a fuel issue
    Go through fuel system thoroughly from tank back:
    Check pickup tube, remove the flap.
    Bypass the fuel selector.
    Possibly new fuel lines, replace filter
    Verify proper check valve operation on fuel vent line.
    Replace pulse lines.

    Not the issue at hand, but have you verified proper PV operation as well?

  3. #3
    I did happen to pull the fuel tank while i was in there and thoroughly cleaned everything before running on the newly rebuilt carbs. I did pull and blow the fuel lines out and installed a fuel filter from the tank to the carbs. I have not yet bypassed the fuel selector but i have run it with the fuel cap loose just in case the tank was being pressurized from the heat of the sun or something wasnt venting properly. I have not checked the PV valves yet. The previous owner did install a new PV motor and they do actuate when i go to crank the engine. Internally i am not sure if they have any issues aside from being able to tell the shaft is rotating between them.

    The pulse lines look in good condition and they are clamped on both ends for each carb.

    With the pressure equalization modification i would think if the fuel system was being pressurized that more than one carb would be showing symptoms.

    I have not removed the flap from the gas tank though. I will take a look into that and trace through the fuel system one more time. I may try another carb kit and go through that carb one more time. Maybe something went wrong with rebuilding that i did not see.

    I will order some parts and try it again next week.
    Thank you,
    Nicholas

  4. #4
    jeffg426690's Avatar
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    Jul 2015
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    Sarasota, FL
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    Opening the gas cap is not helping, it's hurting... Your fuel vent line allows air IN, not out. The fuel system is positive pressure at all times. The vent check valve allows air into the tank, otherwise there would be a vacuum created when the engine is running (which could be part of your issue). Just like pouring gas out of a can without a vent to allow air in...
    If your going to pull the carbs, may as well replace the pulse lines, you can't see an air leak visually.

  5. #5

    Join Date
    Aug 2006
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    Indiana
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    I had a similar problem on a 1200R that I had just rebuilt, carbs and all. Did all the checks like you, took carbs back apart 2 times, reset pop off, finally found the issue: a piece of corrosion( like a flap of aluminum from the side of the fuel passage) in the fuel passageway under the seat and needle was blocking the fuel flow. Carb was only getting enough fuel to fire at idle. Cylinder was cool also, and boat would not go past 25-30. Plug still looked wet because it was too lean to fire so the little fuel made the plug wet like it was rich.

    Quote Originally Posted by nick h View Post
    I have exhausted my thoughts on what could be wrong with this ski. This is a 2001 GP1200R that i purchased from someone who told me the carb tuning was off after installing a D plate. The ski is ridiculously clean and doesn't appear to have many hours on it so i don't believe it to have been abused. When i purchased the ski i was able to get it running on the hose and it was at the end of the season so i figured i would go through the normal stuff before putting it on the water.

    What has been done to the ski:
    D plate
    flame arrestors
    fuel pressure balance modification to carbs (drill returns and place jet in the last carb)
    accelerator pump disabled
    110 pilot jets, 120 main jets, 1.5 needle and seat and 95 gram springs
    45 psi pop off pressure
    1 turn out on low speed and 1.5 turns out on high speed needles
    actual mikuni gasket sets
    new NGK plugs
    oil injection is retained

    The problem:
    cylinder 1 nearest the front of the ski runs cold and when i try to run it on the water the ski goes 20-25 mph and just bogs.

    What i have tried:
    Compression is the same between all cylinders, 160 PSI
    I adjusting needles on the carb to pick the cylinder up, no change.
    I have spark on that cylinder but tried swapping the coil between cylinders in case it wasn't firing under compression
    I tried adjusting pop off pressure thinking maybe the signal was different between the carbs for some reason, made the problematic carb pop off pressure 60 PSI and the others 45 PSI, no change

    The engine will sit and idle within reason but even at idle i can feel the cylinder running noticeably colder than the others. A mechanic at the marina has suggested that a failing crank seal between cylinders may cause a double pulse signal to the carb making for something like this to happen but i have not found anyone to have a similar issue.

    I didn't buy any Chinese junk parts but could a needle and seat have an issue to flood a cylinder to the point it hardly wants to fire?

    Anyone with new thoughts to this issue would be welcome.

    Thank you,
    Nicholas


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